The invention relates to a temperature sensor comprising a sensor element that is arranged in a housing.
There are a number of temperature sensors, based on a variety of different physical measurement principles, on the market for conducting a temperature measurement. Especially popular are electric temperature sensors, for example PTC sensors (positive temperature coefficient sensor) or NTC sensors (negative temperature coefficient sensor), or thermocouples, which have a very simple design and are inexpensive to produce. The actual sensor element can be an electric ohmic resistor, which changes with the temperature. Thermocouples consist of a contact point of two different metals, and this contact point generates a thermal stress when the temperature changes. In the simplest case sensors of this type are produced as a sensor pill with at least two connecting wires. The evaluation of the electric signal is usually performed in an electronic unit, in which the sensor element is a part of a bridge circuit.
For industrial applications it is often necessary to encapsulate the sensor element in a housing. Encapsulation is logical because it constitutes, on the one hand, a reasonable way to install and to make contact with the sensor element. On the other hand, the housing also has to constitute protection for the sensor element, in particular, in rough environments; that is, the sensor pill and the infeed lines are insulated from the environment.
The sensor elements are usually encapsulated in a housing in order to avoid environmental influences, corrosion, etc., for example, due to chemicals or also just due to moisture. Housings of commercially available sensors are made, depending on the application, of metal, ceramic, glass, plastic or other solid materials. Highly stable housings that are made of metal or ceramic are used, in particular, in situations of extremely challenging environmental conditions, and these housings encapsulate the sensor element. When the housing is used in aggressive media or in a high vacuum, it is mandatory that the housing be closed in such a way that it is hermetically tight. In particular, in the ultra high vacuum (UHV) it has to be guaranteed that the sensor will not outgas and contaminate the ultra high vacuum. In this case a hermetically sealed encapsulation is especially necessary.
Since the housing constitutes a thermal insulation of the sensor element against the measuring junction, the temperature that is to be measured is transmitted, delayed by the housing, to the sensor element. Therefore, the housings are made with walls that are as thin as possible, at least at the location of the sensor element, and the sensor element is brought into direct contact with the wall of the housing, in order to produce the best possible heat exchange between the housing and the sensor element. In addition, thermally conductive paste or similar substances exhibiting good heat conduction are often deposited in the housing, in order to improve the thermal contact.
For conventional measuring tasks, for example, in air conditioning technology, in the automobile or in industrial production environments, the accuracy that can be attained with sensors that are constructed in the manner described above is totally adequate.
However, for measuring tasks that require extremely high precision, the past construction technology of these temperature sensors has reached its limits. Since the goal is to achieve the best possible thermal contact with the housing and, in so doing, to thermally connect the sensor element directly to the housing, the result is that mechanical stresses are transferred to the sensor element. These mechanical stresses can be generated, for example, by compressive loads (positive or negative pressure) acting on the sensor. Another source of mechanical stress can be the temperature change itself. The high mechanical stresses can be generated by the different coefficients of thermal expansion of the materials that are used in the sensor. Moreover, even the coupling, for example, by adhesive cementing, or the clamping of the sensor to the object that is to be subjected to a temperature measurement can generate mechanical stress that acts on the sensor element.
If a mechanical stress acts on the sensor element, the result can be a change in the electric resistance and, thus, a change in the measurement signal. In the case of thermocouples any mechanical stress acting on the sensor element can also cause an electric voltage to be generated; and this electric voltage also affects the measurement signal.
The influence is usually in the ppm range and can be ignored in almost all applications. If, however, it is a question of resolving temperatures in the mK range or more specifically to measure reliably, then the mechanical stress results in errors that are larger than the required resolution or more specifically the required accuracy.